PROJECT SUMMARY/ABSTRACT In response to RFA-AI-16-031, we propose ?Keeping it LITE: Exploring HIV Risk in Vulnerable Youth with Limited Interaction,? which will use electronic recruitment to engage young MSM and TGW in an innovative cohort designed to monitor HIV risk and prevention behaviors and explore the socioecological factors that influence behavior and attitudes with minimal engagement. This low-interaction strategy will allow us to describe ongoing infection trends across the Chicago area and characterize individual and community level factors that enhance HIV risk, while minimizing the potential intervention effects of study participation in and of itself. Further, given the rapidly advancing state of HIV prevention technologies, we will use our large, longitudinal cohort of HIV-uninfected youth to monitor PrEP access, uptake and adherence across time. More intensive study of those who acquire HIV while enrolled in the cohort, as well as those ?prevalent positives? at baseline, using innovative social network and phylodynamic analysis, will identify clusters of ongoing active transmission and test the feasibility of novel approaches to target biomedical prevention to those in the highest risk subgroups. As the longitudinal data accumulate, we will be well-poised to design and pilot test virtual strategies for optimal HIV prevention for young MSM and TGW. Our proposed methodologies assure the maximum likelihood of leading to meaningful advances in HIV prevention including the development of tools and strategies that are easily implemented and scalable for export to other communities suffering from similar epidemics of HIV infection. We propose the following specific aims: Aim 1: To enroll and retain a large cohort of 13-34 year-old MSM and TGW (1000/year over 3 year) at high risk of HIV acquisition using tested, novel and cost-effective enrollment strategies through social media and targeted electronic advertisement. Aim 1a: To capture at least 230 HIV seroconversions occurring over the course of the study and 300 previously undiagnosed HIV-infected youth. Aim 1b: To explore behaviors, venues, sociodemographic characteristics and socioecological influences that predict HIV acquisition. Aim 2 (UH3 Stage): To characterize real-world PrEP use among young MSM and TGW enrolled and retained in a longitudinal cohort and explore the individual, provider and community-level predictors of PrEP uptake, PrEP adherence and PrEP discontinuation. Aim 3 (UH3 Stage): To use social network and phylodynamic analysis to identify characteristics of ongoing transmission networks of HIV seroconverters among young MSM and TGW enrolled in the cohort. Aim 3a: To provide data for optimal design and targeting of enhanced partner services and novel biomedical prevention strategies. Aim 4 (UH3 Stage): To describe the HIV care continuum among newly diagnosed young MSM/TGW Aim 4a: Identify individual, network, community and structural predictors of eventual viral suppression.